1. Field of the Invention
The present invention relates to an active material for use in a negative electrode material for a non-aqueous electrolyte secondary battery, such as a lithium ion secondary battery, particularly to silicon particles very useful as a negative electrode active material, and to a method for manufacturing the same.
Moreover, the present invention also relates to a negative electrode for a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery in which the negative electrode material is used.
2. Description of the Related Art
As portable electronic devices and communication devices are rapidly advanced in recent years, non-aqueous electrolyte secondary batteries having a high energy density are strongly desired from the aspects of cost, size and weight reductions.
Silicon is the most promising material to achieve the size reduction and capacity enhancement of the battery since it exhibits an extraordinarily high theoretical capacity of 4200 mAh/g as compared with the theoretical capacity 372 mAh/g of carbonaceous materials that are currently used in commercial batteries.
Silicon is known to take various forms of different crystalline structure depending on the preparation method thereof.
For example, Patent Literature 1 discloses a lithium ion secondary battery using single crystal silicon as a support for a negative electrode active material.
Patent Literature 2 discloses a lithium ion secondary battery using a lithium alloy LixSi (0<=x<=5) with single crystal silicon, polycrystalline silicon or amorphous silicon, where particularly LixSi with amorphous silicon is preferable, and a pulverized crystalline silicon coated with amorphous silicon obtained by plasma decomposition of monosilane is exemplified.
In this case, however, 30 parts of a silicon component and 55 parts of graphite as a conductive agent are used therein as described in Example, the potential battery capacity of silicon cannot be fully taken advantage of.
Patent Literatures 3 to 5 disclose a method in which an electrode current collector having a thin amorphous silicon film accumulated thereon by a vapor deposition method is used as a negative electrode.
There is disclosed a method for suppressing the degradation of cycle performance due to volume expansion by controlling the growth direction, in the method of vapor-phase-growing silicon directly on the current collector (See Patent Literature 6). This method allegedly enables the negative electrode having high capacity and good cycle performance to be manufactured. However, there are problems that cost is high due to a limited production rate, it is difficult to thicken the thin silicon film, and further copper that is the electrode current collector is dispersed to the silicon.
Recently, in view of these, there are disclosed a method for suppressing volume expansion by using silicon particles and restricting the utilization ratio of the silicon battery capacity (Patent Literatures 7 to 9), a method for utilizing grain boundaries in polycrystalline particles as the buffer to volumetric changes by rapidly cooling a melt of silicon having alumina added thereto (Patent Literature 10), polycrystalline particles of mixed phase polycrystals of α- and β-FeSi2 (Patent Literature 11), and high temperature plastic working of a single crystal silicon ingot (Patent Literature 12).
As described above, metallic silicon and silicon alloy with various crystal structures are suggested to utilize silicon as the active material. Any of these has a disadvantage in cost, and the manufacturing method that enables mass synthesis at low-cost has not been suggested yet.